1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a nuclear fuel assembly employing a subassembly having spectral shift-producing rodlets which are adapted to rupture at different times and permit water to enter the rodlets to produce an increase in the water/fuel ratio and thereby an increase in reactivity.
2. Description of the Prior Art
In the conventional design of pressurized water reactors (PWR), an excessive amount of reactivity is designed into the reactor core at start-up so that as the reactivity is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. However, since an excessive amount of reactivity is designed into the reactor core at the beginning of the core life, steps must be taken at that time to properly control it.
One technique to control reactivity is to produce an initial spectral shift which has the effect of increasing the epithermal (low reactivity) part of the neutron spectrum at the expense of the thermal (high reactivity) part. This results in production of fewer thermal neutrons and decreased fission. Then, as fission decreases during extended reactor operation, a reverse shift back to the thermal part of the neutron spectrum at the expense of the epithermal part is undertaken. Such control technique is primarily accomplished through the use of displacer rods. As the name implies, these rods are placed in the core to initially displace some of the moderator water therein and decrease the reactivity. Then, at some point during the core cycle as reactivity is consumed, the displacement associated with these rods is removed from the core so that the amount of moderation and therewith level of reactivity in the core are increased.
One approach contemplated for removing the displacement is to have membranes provided on the ends of the displacer rods which are penetrated at some point in time to allow the rods to be filled with water. A small heating element surrounding a specially indented end cap on the hollow displacer rod is turned on at an appropriate time. The heat weakens the indented part of the end cap to the point where the external water pressure opens the end cap and fills the rod with water.
Another approach used to remove the displacement is the provision of at least one rod in the fuel assembly filled initially with helium or other suitable gas. Then, as reactor operation proceeds, the gas-filled rod expands and increases in length until it engages a spike mounted on the adjacent portion of the top nozzle. The spike pierces the upper end plug of the rod and permits the rod to fill with water. Such approach is described in U.S. Pat. No. 4,371,495 to Marlatt.
Still another approach to displacement removal is to withdraw water displacer rods at the desired time by using a drive mechanism. This approach U.S. Pat. No. 4,432,934 to Gjersten et al.
A further approach to removal of moderator displacement is described in U.S. Pat. No. 4,687,621 to Ferrari. A rod is provided which contains a burnable poison material, such as a boron substance in a form which is water soluble, that generates helium gas. The rod also has a region, such as a rupturable disc-like portion of one end plug, which is specifically fabricated to fail when the helium gas within the rod reaches a given high internal pressure. Also, by varying the initial internal pressure within different groups of the rods in the fuel assemblies, it is possible to have different groups of rods rupture at different times in the core cycle in order to phase-in removal of water displacement in increments.
While all of the above-cited prior approaches operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, a need still exists for an alternate approach to the problem of moderator displacement which is simpler, less costly and still can be tailored to take effect at the desired time during the core cycle.